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Did evolution come before life?

By Bob Holmes

A rudimentary form of natural selection likely existed in the primordial soup even before life arose on Earth. If so, the complex “ecosystem” of prebiotic molecules may have made the eventual arrival of life much more probable.

To examine how this might occur, Martin Nowak and Hisashi Ohtsuki, mathematical biologists at Harvard University, used simple equations to model the growth of such chains of building-blocks.

The model shows that because longer chains require more assembly reactions, they should be much less common than short chains. And if some assembly reactions run faster than others, then chains built from these fast-assembling sequences of building blocks grow to be most abundant.

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Threshold of life

This bare-bones equivalent of natural selection makes the prebiotic soup an interesting place, they say.

“It generates a rich evolutionary dynamic – or what I would want to call a ‘prevolutionary’ dynamic – where you have diversity, you have information, you have complicated chemistry,” says Nowak.

Such a system, full of novel, interacting molecules, would be the ideal milieu to generate a molecule with attributes that would favour the assembly of copies of itself. Nowak’s prebiotic selection could then act to refine this ability by ensuring that better replicators become more common.

At some point, Nowak’s model predicts, the best replicator may get fast and accurate enough to dominate the population, sucking up all the resources and driving all the other prebiotic sequences extinct. This is the threshold of life.

“Ultimately, life destroys pre-life,” says Nowak. “It eats away the scaffold that has built it.”

‘Murky area’

In showing that selection actually precedes the origin of life, and helps to shape it, Nowak helps bridge the gap between nonliving and living systems. In a sense, he says, the prebiotic soup is constantly testing possible replicators, making it much more probable that one might eventually reach the threshold of life.

Nowak’s model helps clarify a murky area of research on prebiotic mixtures, but it offers little direct guidance to experimentalists, says Irene Chen, an origin-of-life researcher also at Harvard.

“The tricky part is figuring out exactly what the relevant chemicals to use are,” she says. “Martin’s model is basically agnostic about that question.”